1. Field of the Invention
The present invention relates to an image forming apparatus such as a color copying machine capable of forming an image on a recording medium with use of recording agents of either two or more than two colors, an image forming method, a program, and a recording medium.
2. Description of the Related Art
In Japanese Unexamined Patent Publication JP-A 62-154971 (1987), in a digital color recording apparatus designed to perform recording with use of recording agents of four colors: C (cyan), M (magenta), Y (yellow), and K (black), on the basis of three-color data obtained through dot development on three-color density data and print-dot positional information, four-color (including a black color) printing data is produced thereby to effect image formation. In this construction, replacement of a composite-color black formed by superimposing three colors: C, M, and Y one after another with a single-color black (K) is conducted in such a manner that the single-color black accounts for almost one half and the C, M, Y-composite color black accounts for the rest.
Moreover, in Japanese Unexamined Patent Publication JP-A 2004-34636, in effecting image formation in an image forming apparatus employing the electrophotographic printing method, in order to avoid that, on the border between a character rendered in a black color and an underlying picture, the color of a recording medium is visible along the outline of the character, so-called Pure Black is used for rendering a character portion in the absence of the under picture, whereas so-called Process Black is used for a black character-under picture overlapping portion, with the under picture and the overlying character processed under the same image-rendering conditions. Process Black is composed of a mixture of C (cyan), M (magenta), and Y (yellow).
In JP-A 62-154971, when there arises misregistration of images that have been formed of the recording agents of four colors: C, M, Y, and K, respectively, in an out-of-register color region around the black-color image formed of the recording agent K is created an image of the C, M, Y-composite color black. This image is created regardless of the presence or absence of a background. Therefore, a background-free area, namely an area around which is formed no image composed of the recording agent, suffers from image quality degradation.
In JP-A 2004-34636, in effecting image formation, the black character-under picture overlapping portion is rendered by Process Black composed of a mixture of C, M, and Y. In this case, the amount of recording agent consumption is large, and also a black character, even as a single piece, is composed of a combination of Pure Black-made portions and Process Black-made portions. This gives rise to a problem of image quality degradation.